To provide a fluid device with a relatively simple structure which can stably function over a long term and convert fluid force and mechanical rotation force with a high degree of effectiveness, particularly a fluid device suitable for a windmill for power generation.

A plurality of blades 9 are provided to a main rotary member around a rotary shaft O so as to be freely rotatable around the rotary shaft 8. Cam followers 13 are provided to the blades 9 at regular intervals L/2 from the rotary shafts 8. The cam follower is abutted to a cam 12 formed in a turntable 2. By so doing, respective blades are changed to a radial direction on one side and a parallel direction on the other side with respect to a wind direction a.